We take a look at a filefish being hailed as the answer to anyone with pest anemones such as Aiptasia and Manjano. But all is not what it seems...

What’s all the fuss about?

The Bristle-tail filefish, (Acreichthys tomentosus) is a fish of the moment. It might be drab, but many enthusiasts are claiming that it’s the solution to Aiptasia in the reef tank.

Aiptasia are small, hardy anemones that spring up in reef tanks, often on live rock but also corals purchased from retailers whose tanks harbour them. They can quickly out-compete and sting neighbouring corals.

You can’t simply dose reef tanks with treatments without affecting the growth of sensitive and desirable inverts such as corals. Bristle-tail filefish are therefore growing in popularity as a natural form of Aiptasia control. Some reports also claim they can have similar effects on Manjano anemones (Anemonia manjano).

How big do they get?

They’re offered for sale very small and are typically just a couple of centimetres long, but could eventually hit 10cm/4”. They’re peaceful and gentle, typically mixing well with most fish.

They’re also pretty and look cute when small. Like other filefish they display fascinating behaviour, especially if there’s more than one in your tank.

What kind of habitats are they found in?

They’re technically not a coral reef species and this may explain why they have not been imported in large numbers before, as they’re likely to occur away from where aquarium fish collectors operate.

Most specimens are caught about 3m/10’ deep in areas ranging from open sand or mud, to sand and seagrass beds, or weedier inshore areas with seaweed, sticks and leaves.

Another common name for tomentosus is Seagrass filefish.

Where does it come from?

Bristle-tail filefish are from the Indo-West Pacific and found between the coast of East Africa and far off the coast of western Australia. They’ve also been found recently in Tonga.

Why are they so interesting?

Filefish have a thickened first dorsal fin spine and a fleshy flap of skin attached to a spinous ray on their underside. During displays with other fish, particularly other filefish, they erect these to increase size and posture to rivals or potential partners.

What do they eat?

There’s a lot of data on the diet of wild Bristle-tails and from stomach analyses we know they feed on a range of items, making them opportunist omnivores. There are no records of corals or anemones in the guts of any fish examined though.

The Bristle-tail is a monacanthid filefish and a member of the Acreichthys genus which contains just two other species; A. hajam and A. radiatus. Acreichthys are a subgenus of Pervagor filefishes but, unlike Pervagor, don’t have a groove in their back to accommodate the chunky dorsal fin spine when not erected.

Don’t filefish have a reputation for being hard to keep?

Filefish can be challenging, so are not for beginners. However, as these are omnivorous and not an obligate corallivore that must have coral polyps in its diet, they’re much easier to cater for. Mark Worboys, of wholesaler Tropical Marine Centre says: “We don’t think they’re particularly difficult, as a filefish will accept other foods, such as frozen brineshrimp, so readily. They appear quite hardy.”

How many should I add?

By keeping just a single filefish you’ll be missing out on some fascinating behaviour and the chance to spawn them. If room, get a pair. Many filefish will pair off without too many problems and simply adding one of each sex is usually all you need to do.

So, it’s possible to sex them?

Yes, but as juveniles they’re sexually monomorphic, which means males and females look almost identical. Bristles develop on males as they approach sexual maturity. Larger, more mature males have bristles that grow longer and bushier. In fully mature specimens they curve back towards the tail.

Females don’t develop bristle patches, so should be easy to sex — though take a photograph of them and zoom in as it’s hard to look at tiny features on this fish as it darts around a tank. Looking from above also helps.

A patch of well-defined bristles on the caudal peduncle is a characteristic of all members of the Acreichthys genus, to which the Bristle-tail belongs. In every species known the bristles are shorter towards the head end and longer at the tail end, so this is a handy characteristic to look for when trying to identify these from similar filefish.

Can they be confused with other species of filefish?

Yes, unfortunately. These are sold when very small and a few other filefish species can be mistaken for the Bristle-tail. Some might not be particularly good reef tank inhabitants and might not eat Aiptasia.

One potential imposter species is the Radial filefish (A. radiatus), a species incredibly similar in appearance and long considered the same species. It has four or more chalky white vertical bars in the middle of the body and a pale anterior bar at the front of the soft dorsal fin. The bars are wider and more obvious in young fish than adults.

A. radiatus also reaches sexual maturity earlier than tomentosus. It’s a smaller fish (up to 5cm/2”) and is typically found on coral reefs, while the Bristle-tail (tomentosus) is larger at 8cm/3.1” and found on sand and seagrass bed habitats. It’s not a fish widely kept so far but may potentially become a better choice to keep than tomentosus.

What exactly are the bristles?

They are modified scales and the bristles themselves are known as spinules. Most specimens between 3-5cm/1.2-2” have three upright spinules that branch out from a central core in the middle of the scale. Smaller specimens have fewer, smaller spinules, while adults over 5cm/2” long have seven to nine that grow from a well-developed core or crest on specific scales.

Although this has similar structures on its flanks, it is a bigger fish that can reach 30cm/12” in length. Its body is covered in much larger tassels than the Bristle-tail. It’s not a sensible choice for the reef tank.

Are they safe with corals and other invertebrates?

Opinions differ. Some report that these fish sometimes have a peck at polyps, but don’t usually eat them. They are developing a reputation for being fairly safe in the reef aquarium, and the one we saw in David Saxby’s tank (see page 72) seemed to be causing few, if any, problems.

However, David’s tank is far from typical and this is still a species that has evolved to feed on sessile invertebrates, so there’s always a risk it might pick at other species.

They don’t appear to cause significant problems in the reef tank while young and in a really big system with rapid coral growth this might not cause too many problems, especially if the filefish are small.

However, it could be noticeable in more modest quarters or if the fish are not kept well fed, and they may need re-homing when they’ve wiped out the Aiptasia — or if they turn their attention to corals, zoanthids or other sessile inverts.

We’d fall short of giving them our recommendation as a reef-safe fish, though.

Mark Worboys, of the marine fish wholesaler Tropical Marine Centre, agrees. “We don’t class the Bristle-tail filefish as a reef-safe species,” he says, “as they may peck at corals and polyps. This is a problem because most people who have Aiptasia problems have them in a reef tank alongside corals.”

Retailer Jason Scott, of The Waterzoo, doesn’t consider them reef-safe either. He says: “We’ve actually had these returned by people who’ve added them to reef tanks following recommendations about their abilities in controlling Aiptasia in the reef tank.

“I believe that they do eat Aiptasia, but they’re not particularly good at it — and they can also eat the other inhabitants in your aquarium.”

Jason feels it ridiculous for some people to recommend them as a reef-safe form of Aiptasia control. He says many other species do the same thing — eat Aiptasia, but also pick at corals and other polyps.

“It’s misleading to market them as Aiptasia controllers. We do have them in the shop sometimes, as I’m a fan of filefish. However, I wouldn’t consider them to be a reef-safe species at all and we don’t sell them for reef tanks.”

Where can I get one?

We’ve seen a few Bristle-tail filefish on sale at UK shops over the past year and demand appears to be growing. They’re available now from most retailers who purchase stock from TMC, who say they’re bringing in fresh batches from Indonesia every week. Expect to pay £14-16 for each small or medium specimen.

Expert Q&A

Matt Clarke explains how to breed this filefish.

Are people actually breeding these?

Yes, a handful have witnessed their Bristle-tails spawning, and one German fishkeeper, Iris Bönig, has successfully raised offspring from hers. If you want a crack at breeding this, or any other marine fish species, look up the excellent Marine Ornamental Fish and Invertebrate Association forums at www.marinebreeder.org.

At what size do they reach sexual maturity?

According to filefish ichthyologists James Tyler and Mark Lange, females begin to develop eggs when about 4.5cm/1.8” long. Most caught at around 4.5-6.5cm/1.8-2.6” have moderately developed eggs and these are fully developed when females hit the 8cm/3.1” mark.

It’s tricky to say when males reach sexual maturity, but if bristle development is an indicator they start to approach it around the 4cm/1.6” point and are fully mature by 5-7cm/2-2.8” when males have ‘retrorse’ bristles which point backwards. Well-developed bristles are thought to be a good sign of spawning age.

How do they breed?

Females get plumper prior to spawning. Unlike many other marine fish, which lay tiny pelagic eggs that float in the plankton, filefish are egg depositors and lay spherical, adhesive eggs usually near the substrate.

Iris says her Bristle-tail filefish had laid their slightly adhesive masses of around 300 1mm eggs in the sand and around clumps of algae, and she observed the female defending the clutch from other fish. Filefish eggs often have oil globules in the egg and, according to Iris, Bristle-tail eggs are no exception.

How were the offspring raised?

Iris’s eggs hatched after three days in a tank with water held at 25°C/77°F. The fry were about 2.7mm long after hatching.

Matt Pedersen, one of the founding members of the Marine Ornamental Fish and Invertebrate Association, says it’s difficult to advise on breeding techniques as, so far, only he and Iris have been successful with any filefish species!

Iris fed hers on brineshrimp nauplii for the first 12 days. By day 15 she found hers in the juvenile stage, after which they could be weaned on to other foods.